1. Field of the Invention
The present invention relates to an optical pick-up unit, and more particularly to one provided with a holder holding an objective lens etc. supported by spring members, the object lens being movable in both of a focusing direction and a tracking direction.
2. Description of the Prior Art
An optical pick-up unit of an information recording/reading apparatus for recording or reading information from a disc-shaped recording medium (hereinafter referred to as the disc) is carried on a carriage which is rectilinearly movable in a radial direction of the disc. In general, a rotary motor is used as a prime mover for the carriage. A torque of the rotary motor is transmitted to the carriage through a suitable gear-reducer mechanism, for example such as one constructed of a rack and a pinion.
In the information recording-reading apparatus, since the rotary motor has a limitation in its responsibility, the apparatus has a limitation in its quick-access ability for driving the optical pick-up unit at a high speed to position the unit on a desired recording track of the disc. Consequently, there is a strong demand for a quick-access information recording/reading apparatus having an optical pick-up unit. In recent years, in order to realize such quick-access information recording/reading apparatus, there is employed a linear motor as a means for driving the optical pick-up unit of the apparatus, the linear motor being excellent in responsiveness.
As shown in FIG. 1, in the optical pick-up unit, a holder 1 holding therein an objective lens (not shown) is supported by a conventional biaxial-support structure constructed of a base mount 3 and four pieces of wire-like spring members 2a, 2b, 2c and 2d which are parallel to one another. The biaxial-suport structure supports the holder 1 through the spring members 2a, 2b, 2c and 2d so as to be movable in two perpendicular directions, i.e., in an optical-axis direction "A" or focusing direction and in a direction "B" or tracking direction perpendicular to the optical axis direction "A". The objective lens functions to converge an optical beam passed through a hole 3a formed in the base mount 3 toward an optical disc (not shown).
As described above, in the conventional biaxial-support structure suporting the holder 1 through the four pieces of the spring members 2a, 2b, 2c and 2d parallel to one another, a minimum resonant frequency f0 of the holder 1 in the focusing direction "A" is the same as that of the holder 1 in the tracking direction "B". However, since a focusing servo-mechanism of the optical pick-up unit moves into its retracted position in operation, it is difficult to set the minimum resonant frequency f.sub.o of the holder 1 in the focusing direction "A" at a high level. Consequently, in the conventional biaxial-support structure, it is natural to set the minimum resonant frequency of the holder 1 at a low level or low frequency.
In case that a linear motor is employed as a means for driving the optical pick-up unit having the biaxial-support structure so as to realize a quick-access information recording/reading apparatus having the optical pick-up unit, an external force exerted by the carriage acts on the holder 1 through the wire-like spring members 2a, 2b, 2c and 2d since the carriage carrying the optical pick-up unit is operated under a condition that the focusing servo is actuated while the tracking servo is not actuated. In this case, the minimum resonant frequency f.sub.o of the holder 1 in the tracking direction is low. This means that an assembly of the spring members 2a, 2b, 2c and 2d of the optical pick-up unit is low in its spring constant in the tracking direction. Consequently, the objective lens in the holder 1 is moved to a position slightly deviated from its neutral position when the quick-access operation of the optical pick-up unit is conducted, whereby a positioning error is produced at this time. Due to the existence of such positioning error or a positioning error caused in a tracking-error detection system employing a push-pull method and the like, the conventional optical pick-up unit is poor in convergency on a desired track of the disc to take much access time therefor.